1 00:00:00,000 --> 00:00:02,896 I hope you enjoyed last week's world wind tour of physics. 2 00:00:02,896 --> 00:00:06,292 But, I bet that you're ready to start thinking about astronomy again. 3 00:00:06,292 --> 00:00:09,638 We're going to do that now, we're going to take all tools we acquired and 4 00:00:09,638 --> 00:00:12,734 start applying them to understanding the rest of the universe. 5 00:00:12,734 --> 00:00:16,280 It seems fitting to start with our local neighborhood, the solar system. 6 00:00:16,280 --> 00:00:19,732 This is both, because our local neighborhood is very important. 7 00:00:19,732 --> 00:00:23,017 It's where we live. And also because in a way, we know a lot 8 00:00:23,017 --> 00:00:27,248 more about the solar system than about anywhere in this, rest of the universe, 9 00:00:27,248 --> 00:00:31,925 because over the past few decades we've been able to send spacecraft robotic 10 00:00:31,925 --> 00:00:35,266 spacecraft to visit many of the objects in the solar system. 11 00:00:35,266 --> 00:00:38,551 Allowing us to verify the predictions we had made based on ground base 12 00:00:38,551 --> 00:00:42,393 observations that are understanding of physics and compare them to actual 13 00:00:42,393 --> 00:00:45,456 measurements. So we will be able to verify a lot of our 14 00:00:45,456 --> 00:00:48,240 understanding of what goes on in the sky directly. 15 00:00:48,240 --> 00:00:52,533 For both of those reasons, we're going to spend the next week studying the solar 16 00:00:52,533 --> 00:00:55,210 system. I should point out at the beginning that 17 00:00:55,210 --> 00:00:59,726 there are at least two aspects of the science of the solar system, that I will 18 00:00:59,726 --> 00:01:02,124 not be able to give the appropriate time to. 19 00:01:02,124 --> 00:01:05,955 This includes A, the amazing and fascinating history of space travel, in 20 00:01:05,955 --> 00:01:10,427 our exploration of the solar system. that will be given short thrift because 21 00:01:10,427 --> 00:01:14,313 we don't have time to put into it. And the other aspect is the intriguing 22 00:01:14,313 --> 00:01:18,360 and exciting question of, the possibility for the existing of life elsewhere. 23 00:01:18,360 --> 00:01:23,931 in the solar system or beyond and this is not something about which I know very 24 00:01:23,931 --> 00:01:27,300 much and it's not the center of focus of this class. 25 00:01:27,300 --> 00:01:32,288 So we will be discussing the physics of the solar system leaving the biology 26 00:01:32,288 --> 00:01:35,009 aside. So with that said, what is the solar 27 00:01:35,009 --> 00:01:37,989 system? Well, primarily the solar system is all 28 00:01:37,989 --> 00:01:43,430 of the objects in bound orbits around the Sun and most of that is a star, it's the 29 00:01:43,430 --> 00:01:48,442 sun To a great extent, 99.9% of the mass of the solar system is in this 30 00:01:48,442 --> 00:01:51,971 average-sized, main sequence star we call our sun. 31 00:01:51,971 --> 00:01:58,019 And this star dominates the solar system in more ways than containing all of the 32 00:01:58,019 --> 00:02:00,654 mass. In addition to dominating the mass of the 33 00:02:00,654 --> 00:02:05,150 solar system, the sun is extends its influence throughout the solar system in 34 00:02:05,150 --> 00:02:07,587 two other ways. One, we talked about last week. 35 00:02:07,587 --> 00:02:10,675 Is the radiation. The black body radiation with which the 36 00:02:10,675 --> 00:02:13,600 sun glows. that is the source of our light and heat 37 00:02:13,600 --> 00:02:16,200 here on earth and elsewhere in the solar system. 38 00:02:16,200 --> 00:02:21,680 In addition, as we'll see as part of the processes that go in a star, the outer 39 00:02:21,680 --> 00:02:27,092 atmosphere of the sun is a very, active place, and it is constantly emitting a 40 00:02:27,092 --> 00:02:31,964 stream of charged particles called the solar wind that leave at high, the sun at 41 00:02:31,964 --> 00:02:35,550 high speed and propagate throughout the solar system. 42 00:02:35,550 --> 00:02:39,313 We will discuss this when we talk about the sun next week. 43 00:02:39,313 --> 00:02:44,245 But this is a nontrivial flux of high-energy particles away from the sun. 44 00:02:44,245 --> 00:02:48,527 And as we'll see it, it, it has lots of effects in the solar system. 45 00:02:48,527 --> 00:02:53,848 To give you a sense, every 150,000,000 years the sun loses a full earth mass to 46 00:02:53,848 --> 00:02:57,612 this solar wind. So there is this constant stream of stuff 47 00:02:57,612 --> 00:03:00,794 coming out of the sun. Okay, we know about the sun. 48 00:03:00,794 --> 00:03:04,080 We'll discuss the sun next week when we talk about the stars. 49 00:03:04,080 --> 00:03:08,448 What else is out there? Well primarily what we think of is the 50 00:03:08,448 --> 00:03:13,663 solar system, is the eight planets. The sun is orbited by eight large, round 51 00:03:13,663 --> 00:03:19,524 objects, in slightly eccentric but almost exactly circular orbits of various sizes, 52 00:03:19,524 --> 00:03:24,674 ranging from point four astronomical units. Mercury is the closest plane to 53 00:03:24,674 --> 00:03:29,206 the sun, all the way out to 30 astronomical units, or two orders of 54 00:03:29,206 --> 00:03:32,913 magnitude, of orbital radius all the way up to Neptune. 55 00:03:32,913 --> 00:03:38,338 And we see in the this nice image that there's a nice cluster of very small 56 00:03:38,338 --> 00:03:42,290 inner solar system orbits and then the more 57 00:03:42,290 --> 00:03:46,985 widely spaced outer solar system orbits and essentially the space between the 58 00:03:46,985 --> 00:03:50,778 planets is mostly empty. So, roughly, it's the sun with a planets 59 00:03:50,778 --> 00:03:55,112 orbiting it and the orbits are distinguished not only by being circular 60 00:03:55,112 --> 00:03:58,363 but note in this image, they are almost all in a plane. 61 00:03:58,363 --> 00:04:03,119 So there are no planets that orbit the sun this way or that orbit the sun that 62 00:04:03,119 --> 00:04:05,767 way. They all orbit in the same sense, in the 63 00:04:05,767 --> 00:04:08,300 same plane. Clearly crying for explanation. 64 00:04:08,300 --> 00:04:13,128 When one observes the sun as Galileo did when notices that the sun in fact spins 65 00:04:13,128 --> 00:04:15,844 about an axis. And the sun's axis, surprise is 66 00:04:15,844 --> 00:04:19,947 perpendicular to the ecliptic. So the sun, too, rotates about its axis 67 00:04:19,947 --> 00:04:25,401 in the same sense as do the planets. the planets themselves come in two very 68 00:04:25,401 --> 00:04:28,845 distinct families. There are the inner planets, Mercury, 69 00:04:28,845 --> 00:04:33,754 Venus, Earth and Mars, the outer planets, Jupiter, Saturn, Uranus and Neptune and 70 00:04:33,754 --> 00:04:37,389 they're different in as many ways as you can think of. 71 00:04:37,389 --> 00:04:42,107 the inner planets are dense, as we shall see, as dense as rock or denser. 72 00:04:42,107 --> 00:04:46,954 They are small, this image is not to scale, the left-hand side is to scale, so 73 00:04:46,954 --> 00:04:51,290 that you see that Mars and Mercury are smaller than Earth and Venus. 74 00:04:51,290 --> 00:04:56,163 the right hand side is the scale, which shows you that Jupiter is the largest. 75 00:04:56,163 --> 00:05:00,135 But, between the right and left hand side is about a factor of ten. 76 00:05:00,135 --> 00:05:04,226 In size, Jupiter in fact is eleven times as large in radius as Earth, 77 00:05:04,226 --> 00:05:08,407 if we put the two figures to scale, and either we wouldn't see the inner planets 78 00:05:08,407 --> 00:05:12,004 or the outer planets wouldn't fit. So, you have to draw a very sharp line 79 00:05:12,004 --> 00:05:15,451 between the right and the left. The outer planets are not just bigger, 80 00:05:15,451 --> 00:05:19,517 they are also far less dense. and their composition is different, 81 00:05:19,517 --> 00:05:22,444 as we will see, the inner plants are rocky. 82 00:05:22,444 --> 00:05:25,762 Their composition is silicates and iron and nickel. 83 00:05:25,762 --> 00:05:32,170 The outer planets are mostly hydrogen, helium and Isis and lighter elements, 84 00:05:32,170 --> 00:05:34,058 larger planets. Why this dichotomy? 85 00:05:34,058 --> 00:05:37,548 Certainly an interesting question. Okay, we have eight planets. 86 00:05:37,548 --> 00:05:41,897 The solar system is not just the sun and eight planets, there's other things 87 00:05:41,897 --> 00:05:44,071 floating out there. What else is there? 88 00:05:44,071 --> 00:05:47,791 Well, around many of the planets are objects that are captured in 89 00:05:47,791 --> 00:05:52,025 gravitational Keplerian orbits about the planets as they orbit the sun. 90 00:05:52,025 --> 00:05:54,371 They're called moons, we know of our moon. 91 00:05:54,371 --> 00:05:59,213 There are a total of about a 150 Moons in the solar system most of them orbiting 92 00:05:59,213 --> 00:06:03,642 Jupiter and Saturn, but all of the planets except Mercury and Venus have 93 00:06:03,642 --> 00:06:06,718 moons. the outer planets tend to have many more 94 00:06:06,718 --> 00:06:11,025 moons than the inner planets and some of these moons are planet sized. 95 00:06:11,025 --> 00:06:16,008 Our moon is about almost the size of Mercury, some of the moons of Jupiter are 96 00:06:16,008 --> 00:06:18,961 very large. Again notice that all of these large 97 00:06:18,961 --> 00:06:22,225 objects, like the planets. All have the same shape. 98 00:06:22,225 --> 00:06:26,898 So, they're all spherical. We want to understand why is everything 99 00:06:26,898 --> 00:06:29,449 round. and where do these moons come from and 100 00:06:29,449 --> 00:06:31,770 why do they orbit the planets and not the sun. 101 00:06:31,770 --> 00:06:36,750 in addition to moons orbiting the planets, there are these beautiful 102 00:06:36,750 --> 00:06:39,992 structures of rings. It turns out that, as we found in the 103 00:06:39,992 --> 00:06:44,143 twentieth century, all of the giant planets have ring systems surrounding 104 00:06:44,143 --> 00:06:46,930 them and we'll see that that's a ubiquitous fact. 105 00:06:46,930 --> 00:06:50,172 But Saturn's rings are certainly a spectacular exception. 106 00:06:50,172 --> 00:06:54,949 there's a reason why Galileo discovered the rings of Saturn and it was not until 107 00:06:54,949 --> 00:06:58,930 the twentieth century that we found the other giant planets had rings. 108 00:06:58,930 --> 00:07:03,163 and we'll want to understand what this structure is, where it came from, and 109 00:07:03,163 --> 00:07:06,339 what it tells us. Looking at the moons and the planets of 110 00:07:06,339 --> 00:07:10,740 the solar system we see another property that they all share, which tells us. 111 00:07:10,740 --> 00:07:14,065 Something about what goes on in the solar system. 112 00:07:14,065 --> 00:07:18,605 And that is, that they all seem to have suffered a little bit of damage. 113 00:07:18,605 --> 00:07:23,594 this image is an image of Mercury, and you see that it looks kind of like the 114 00:07:23,594 --> 00:07:26,407 moon. Because like the moon, it's pock-marked 115 00:07:26,407 --> 00:07:30,308 with craters, the results of collisions some time in its past. 116 00:07:30,308 --> 00:07:34,529 This image shows you that the Earth is not immune to such attacks. 117 00:07:34,529 --> 00:07:39,390 This famous crater in Arizona was formed by an impact only 50,000 years ago. 118 00:07:39,390 --> 00:07:44,469 And so, there have been and are impacts onto the Earth and this feature of 119 00:07:44,469 --> 00:07:48,175 cratering is one of the most ubiquitous things we see. 120 00:07:48,175 --> 00:07:54,284 Here's our familiar moon, with this is the nearside of the moon and the familiar 121 00:07:54,284 --> 00:08:00,050 cratered shape of Tycho and so on shows us that the moon too has taken the brunt 122 00:08:00,050 --> 00:08:03,413 of many hits. Here is Ganymede, Jupiter's moon and 123 00:08:03,413 --> 00:08:08,355 Ganymede too has had the impacts. What caused this, is it still happening, 124 00:08:08,355 --> 00:08:12,680 are we about to get hit? All questions we'll want to understand. 125 00:08:12,680 --> 00:08:17,799 So we have planets, moons, rings, something that crashes into things and 126 00:08:17,799 --> 00:08:20,579 creates craters. What else is out there? 127 00:08:20,579 --> 00:08:26,650 Well turns out that's all the big stuff but there are smaller objects out there 128 00:08:26,650 --> 00:08:30,488 The main collection of small objects is called asteroids. 129 00:08:30,488 --> 00:08:35,809 this is a picture of a particularly pretty one, this is the asteroid Eros 130 00:08:35,809 --> 00:08:38,974 close-up shot. And we see two things about Eros. 131 00:08:38,974 --> 00:08:43,958 One is that, like everything else in the solar system, it has suffered some 132 00:08:43,958 --> 00:08:48,402 collisions, it's cratered. But Eros is not round, it's shaped like a 133 00:08:48,402 --> 00:08:50,760 potato. And in general asteroids, 134 00:08:50,760 --> 00:08:55,437 do not necessarily follow the pattern we've seen where everything is spherical. 135 00:08:55,437 --> 00:08:59,818 We'll have to understand why it is that things can come in various shapes. 136 00:08:59,818 --> 00:09:03,903 There's this relatively densely populated region of the solar system. 137 00:09:03,903 --> 00:09:08,403 There's about a tenth of an Earth mass orbiting in the asteroid belt and in 138 00:09:08,403 --> 00:09:13,317 addition, for example in Jupiter's orbit, there are these so-called two groups of 139 00:09:13,317 --> 00:09:18,053 so-called Trojan asteroids that orbit in Jupiter's orbit but 60 degrees east or 140 00:09:18,053 --> 00:09:20,540 west of the planet and they orbit because, 141 00:09:20,540 --> 00:09:23,467 It's a Kepler orbit with the same period as Jupiter. 142 00:09:23,467 --> 00:09:26,475 Another thing we'll have to pay attention to. 143 00:09:26,475 --> 00:09:30,657 The solar system, we think, can not possible end at Neptune. 144 00:09:30,657 --> 00:09:36,425 There are objects that are a bit beyond Neptune's orbit, and our best hint about 145 00:09:36,425 --> 00:09:41,255 them comes first from comets. Here's a pretty view of Haley's comet 146 00:09:41,255 --> 00:09:46,271 from it's 1910 apparition. And it turns out that comets in their, 147 00:09:46,271 --> 00:09:52,170 cometary orbits, where they approach the sun, cannot survive billions of years. 148 00:09:52,170 --> 00:09:57,227 We'll talk about that later. And in order to explain the frequency 149 00:09:57,227 --> 00:10:03,280 with which comets are observed entering the inner solar system in the plane of 150 00:10:03,280 --> 00:10:07,034 the ecliptic. we conjecture the existence of a 151 00:10:07,034 --> 00:10:11,542 collection of objects. Orbiting beyond Neptune's orbit. 152 00:10:11,542 --> 00:10:17,375 It's called the Kuiper Belt, a disc-shaped distribution with it is 153 00:10:17,375 --> 00:10:23,938 assumed tens of thousands of objects, hundreds of which are the size of the 154 00:10:23,938 --> 00:10:28,556 moon or larger. Pluto would be an example of one of these 155 00:10:28,556 --> 00:10:30,420 Kuiper Belt object. And. 156 00:10:30,420 --> 00:10:36,000 these are objects which when perturbed from their orbits, form the source of the 157 00:10:36,000 --> 00:10:40,997 short and intermediate range comets that we see orbiting in the plane of the 158 00:10:40,997 --> 00:10:46,124 ecliptic and it turns out that in order to explain the frequency with which we 159 00:10:46,124 --> 00:10:51,315 see comets, we can predict the density of objects that must be out in the Kuiper 160 00:10:51,315 --> 00:10:55,891 belt. the Kuiper belt might extend out to as 161 00:10:55,891 --> 00:11:00,850 much as 100 astronomical units away from the sun. 162 00:11:00,850 --> 00:11:08,846 much farther away, it is conjectured that way past the orbits of the planets, is 163 00:11:08,846 --> 00:11:08,846 this spherically symmetric halo of object. 164 00:11:08,846 --> 00:11:13,603 the Oort cloud, it might have an inner Oort loud along the disk or the Hill 165 00:11:13,603 --> 00:11:19,980 cloud but these objects orbit the sun in a highly eccentric, 166 00:11:19,980 --> 00:11:24,779 Orbits outside the plane of the ecliptic, and they form the source for out long 167 00:11:24,779 --> 00:11:27,349 period comets. This is our collection. 168 00:11:27,349 --> 00:11:33,341 the first question you want to ask is What's it made of? And we can answer that 169 00:11:33,341 --> 00:11:36,965 rather directly. the answer of course to, "What is the 170 00:11:36,965 --> 00:11:41,840 solar system made of?" is mostly the answer of the question, "What is the Sun 171 00:11:41,840 --> 00:11:44,740 made of?" because 99.9% of the mass is the Sun. 172 00:11:44,740 --> 00:11:48,579 Of the.1 percent that is not the sun, 90% of the rest is Jupiter. 173 00:11:48,579 --> 00:11:53,759 and Saturn and we have here a plot of the abundances of various atomic species in 174 00:11:53,759 --> 00:11:58,634 the solar system, which roughly mirrors the abundance of the same atomic species 175 00:11:58,634 --> 00:12:02,474 throughout the universe. So when we see what the solar system's 176 00:12:02,474 --> 00:12:06,130 made of, we're really answering what the universe is made of. 177 00:12:06,130 --> 00:12:11,127 We'll come back and try to understand this later in the class but for now it's 178 00:12:11,127 --> 00:12:16,063 worth the observation and these are the relative numerical abundances of various 179 00:12:16,063 --> 00:12:18,984 species of atoms, organized by atomic number. 180 00:12:18,984 --> 00:12:24,221 And what you need, the, the scale is kind of random, but it's a logarithmic scale. 181 00:12:24,221 --> 00:12:29,876 So that the fact that helium registers one less than hydrogen means that there's 182 00:12:29,876 --> 00:12:33,366 about one helium atom for each ten hydrogen atoms. 183 00:12:33,366 --> 00:12:39,091 There's a factor of ten less helium than hydrogen and together, because everything 184 00:12:39,091 --> 00:12:44,842 is down by a factor of 10,000, they comprise almost the entire content of the 185 00:12:44,842 --> 00:12:49,307 solar system by mass, because a helium atom weighs 186 00:12:49,307 --> 00:12:55,919 four times as much as a hydrogen atom. that works out to about 70% hydrogen, 28% 187 00:12:55,919 --> 00:13:01,336 helium and 2% the rest. everything else is trace amounts, down by 188 00:13:01,336 --> 00:13:07,151 a factor, as I said, of at least 10,000, from the abundance of hydrogen and 189 00:13:07,151 --> 00:13:10,657 helium. And this is a sobering note when you 190 00:13:10,657 --> 00:13:16,106 remember that we and essentially, our entire planet is made up mostly of these 191 00:13:16,106 --> 00:13:20,198 trace left-over materials. Astronomers tend to call anything that 192 00:13:20,198 --> 00:13:24,478 is, heavier than lithium. So anything past the first three or four 193 00:13:24,478 --> 00:13:27,500 elements. Metals, carbon is not a metal in the 194 00:13:27,500 --> 00:13:31,592 chemical sense of the word. But in astronomy, heavier elements are 195 00:13:31,592 --> 00:13:35,054 all called metals. So we say that the mass of the solar 196 00:13:35,054 --> 00:13:39,020 system and the mass of the universe, essentially, has 2% metals. 197 00:13:39,020 --> 00:13:42,890 But mostly, it's a world of hydrogen and helium. 198 00:13:42,890 --> 00:13:46,783 That's our quick survey. this raises many questions. 199 00:13:46,783 --> 00:13:52,515 I've listed some of them here and we'll try to see how many of them we can answer 200 00:13:52,515 --> 00:13:56,630 and how many more we can raise as we get into more detail 201 00:13:56,630 --> 00:14:00,325 Why are all the planets orbits circular? Why are they all in a plane? 202 00:14:00,325 --> 00:14:03,422 Why are the comet orbits neither circular nor in a plane? 203 00:14:03,422 --> 00:14:07,498 Why are all the planets and the large moons spherical objects and if so, why 204 00:14:07,498 --> 00:14:10,704 are the asteroids not? Why are there two kinds of different 205 00:14:10,704 --> 00:14:13,312 planets? Why are there inner planets of one kind 206 00:14:13,312 --> 00:14:16,736 and outer planets of a completely different species of objects? 207 00:14:16,736 --> 00:14:20,377 why aren't the asteroid is a planet? There's clearly stuff there. 208 00:14:20,377 --> 00:14:24,289 there's room between Mars and Jupiter, why didn't a planet form there? 209 00:14:24,289 --> 00:14:28,691 Why didn't asteroid belts form elsewhere? Why didn't the rest of the solar system 210 00:14:28,691 --> 00:14:32,046 coalesced into planets? It's not a science question, but it's 211 00:14:32,046 --> 00:14:34,960 always asked. What happened to Pluto to get it demoted 212 00:14:34,960 --> 00:14:38,431 from planet to dwarf planet? What's a dwarf planet anyway? 213 00:14:38,431 --> 00:14:42,169 what are rings? Why does Saturn come equipped with these 214 00:14:42,169 --> 00:14:45,338 brilliant special rings? What made all the craters? 215 00:14:45,338 --> 00:14:49,140 Where did it go, is it coming back? Are we about to get 216 00:14:49,140 --> 00:14:53,562 it hit with some big impact. Remember we did our homework last week 217 00:14:53,562 --> 00:14:56,731 and we saw that this can be a significant event. 218 00:14:56,731 --> 00:15:01,879 Why do comets sometimes leave their comfortable orbits out in the Kuiper belt 219 00:15:01,879 --> 00:15:05,642 and the Oort cloud and come diving into the solar system? 220 00:15:05,642 --> 00:15:10,791 Why do some asteroids move from the asteroid belt into orbits that bring them 221 00:15:10,791 --> 00:15:14,487 very near earth? We keep hearing of near-earth asteroids 222 00:15:14,487 --> 00:15:17,260 and their potential for collisions. And, 223 00:15:17,260 --> 00:15:21,433 if orbits can change, if something can move from a comfortable orbit out of the 224 00:15:21,433 --> 00:15:24,180 Oort cloud and come crashing into the solar system. 225 00:15:24,180 --> 00:15:28,585 Will the planets orbits change? Will the solar system remain in its 226 00:15:28,585 --> 00:15:32,004 current configuration? Have planetary orbits changed? 227 00:15:32,004 --> 00:15:35,226 Was the solar system always the way it is now? 228 00:15:35,226 --> 00:15:37,772 And, underlying all of this, the question of 229 00:15:37,772 --> 00:15:41,108 where did all of this come from, and when did that happen? 230 00:15:41,108 --> 00:15:45,556 And we'll find that starting from the end, in fact, is the way we're going to 231 00:15:45,556 --> 00:15:50,004 go in the next clip, we're going to figure out when it started and from that, 232 00:15:50,004 --> 00:15:54,393 we'll try to go on to how it started. And that will try to generate as many 233 00:15:54,393 --> 00:15:56,969 answers as we can to all of these questions.